1. Field of the Invention
The present invention relates generally to nuclear steam generator maintenance devices, and more particularly to a geared track installable in the annular space of a steam generator for providing a stable base for operating maintenance equipment.
2. Description of the Prior Art
Steam generators are used in nuclear power stations for heat exchange purposes in the generation of steam for driving turbines. A primary fluid, heated by the core of the reactor, is led through a bundle of U-type or once-through tubes in the steam generator. A secondary fluid, generally water, is fed into the space around the tubes and receives heat from the tubes to provide a continuous steam generation cycle. Due to the constant high temperature and severe operating environment of the steam generator, internal maintenance procedures are required for the generator. One of the most common procedures is the removal of sludge deposits from the lower tubesheet region. The sludge buildup on the tubesheet and tubes reduces the heat transfer efficiency of the tubes and can cause corrosion. Therefore, it is preferable that the tubes and tubesheet be periodically cleaned to remove the sludge.
Many steam generators have a square-pitch tube configuration which have well-defined channels between the tubes, as described hereinafter. These can be cleaned rather easily with a sludge lance apparatus from the tubelane between the semi-circular tube bundles. The lance is equipped with a series of discharge nozzles which spray streams of high pressure water at the area between the tubes to dislodge the sludge. This type of equipment is quite stable since it may be bolted directly to the steam generator frame.
For example, Lee, U.S. Pat. No. 5,065,703, discloses a system for lancing sludge deposits from within a bundle of square-pitch steam generator tubes. A flexible lance is moved down the tubelane and inserted in the inter-tube lanes for sludge removal using pressurized fluid.
Ruggieri, U.S. Pat. No. 5,036,871 discloses a similar flexible lance system operative from the tubelane. A support rail is provided along the tubelane for supporting and driving the lance.
Most recent generators having been replacing the square-pitch tube configuration with a triangular-pitch. This configuration enables the generator to contain more tubes per unit area, and thus increases heat transfer. However, the increased density of the tubes makes it more difficult to clean between the tubes. Other techniques have been developed to provide more effective lancing of these newer systems.
Klahn et al., U.S. Pat. No. 4,757,785, discloses a lance assembly for cleaning the tube areas of a once-through steam generator (OTSG) inaccessible from the tubelane. A track is assembled in the annular space between the OTSG outer shell and a circular shroud which surrounds the tube bundle. A remotely controlled movable lancing apparatus is driven along the track for cleaning the tubes through windows in the shroud.
Previous devices which attempted to clean the tubes from the annular space tend to be unstable. These devices are usually self-propelled, or cable driven, and are top-heavy. When the water pressure to the lance is turned on, the apparatus leans back due to the force of the water, and the streams of water cannot be adequately controlled or aimed between the triangular tubes where the sludge is located. The distance between the tubes is very small and the lance is difficult to align even under ideal circumstances, if the tubes were visible. But in this case, the tubes cannot be seen as they are located in inaccessible areas. To overcome this difficulty, it is possible to locate a camera in the tube lane, or near the lance. The camera quickly becomes covered by water and is of limited utility.
A further difficulty with previous systems is that they are limited to a single purpose, i.e. sludge lancing. None of the prior art is directed toward providing a generalized stable platform for a steam generator maintenance apparatus, i.e. tube repair, parts retrieval, etc. Furthermore, none of the prior systems provided a means for providing an accurate position determination around the steam generator without the use of extraneous optical equipment.